Openable thermal printer

ABSTRACT

A thermal printer for printing on paper (26), the printer comprising a thermal print head (2) that co-operates with a paper drive roll (15) by bearing thereagainst, in which the print head is carried resiliently by a fixed chassis portion (1) while the drive roll (15) is carried by a moving chassis portion (13) hinged to the fixed chassis (1) and forming a cover giving access to the compartment in the printer for containing the roll (12) of paper, characterized in that, when the cover is closed, the zone of contact between the paper drive roll (15) and the row (4) of heating points of the print head (2) is situated above the diameter (14) of said roll that passes through the hinge between the moving chassis (13) and the fixed chassis (1).

The present invention relates to a thermal printer in which insertingand removing the print medium are facilitated.

BACKGROUND OF THE INVENTION

The print medium for thermal printing is generally in the form of a rollof sheet material which is paid out beneath a print head by means of adrive roll (or capstan) and which is then cut into lengths to constituteseparate sheets (or receipts, etc.).

One of the main drawbacks of that type of apparatus lies in thedifficulty of threading the leading end of a roll through the print headand between the blades of the device for cutting the sheet into lengths.

Printers have already been proposed in which the print medium isinstalled from the side, i.e. after removing a cover that isperpendicular to the axes of the capstan and of the roll so that themedium can be slotted in parallel to said axes.

Printers also exist in which the print head is separated from thecapstan so that they are moved apart from each other parallel to theaxis of the capstan. The capstan is then secured to a chassis componentthat pivots on a fixed component carrying the print head. The pivotingcover moves in relatively complex manner in order to avoid rubbingcontact between the capstan and the head when they are being broughttogether or moved apart. The capstan support therefore begins itsseparation movement substantially perpendicularly to the head and thenmoves parallel to the head for disengagement purposes once the capstanis no longer in contact with the head.

The present invention seeks to simplify the structure of thermalprinters that are capable of being opened in order to facilitateinsertion of the strip of paper; it also has the advantage of beingsimpler to use than existing printers.

SUMMARY OF THE INVENTION

To this end, the invention therefore provides a thermal printer forprinting on a thermal paper type medium, the printer comprising athermal print head that co-operates with a paper drive roll by bearingthereagainst, in which the print head is carried resiliently by a fixedchassis portion while the drive roll is carried by a moving chassisportion hinged to the fixed chassis and forming a cover giving access tothe compartment in the printer for containing the roll of paper.According to one of the characteristics of the invention, when the coveris closed, the zone of contact between the paper drive roll and the rowof heating points of the print head is situated behind the diameter ofsaid roll that passes through the hinge between the moving chassis andthe fixed chassis, where "behind" is as seen in the closure direction ofthe cover about its hinge. This disposition makes it possible to ensureenough pressure to hold the paper properly against the capstan, theprint head forming a resilient latch on closure of the cover.

The portion of the print head situated in front of the row of heatingpoints is shaped to offer a cam-forming surface for displacing the headagainst the force of a resilient member when the backing roll goes pastin the cover-closing direction. In contrast to what could have beenfeared, this direct contact (with the strip of paper in between) givesrise to no premature damage or wear of the head or of the capstan,thereby making it possible to avoid installing cam devices external tothe capstan and to the print head. The capstan rolls without slippingover said (paper-covered) cam surface and therefore without producingany rubbing between the head and the paper that could be driven by theroll. The absence of any complex cam mechanism means that the total bulkof the device is as small as possible.

In order to obtain maximum benefit from an opening thermal printer, theinvention also proposes adding thereto a paper cutter member that islikewise capable of opening. To do this, the fixed chassis carries therotary blade of the cutting member downstream from the print head in thepaper travel direction, with the stationary blade being carried by thecover downstream from the backing roll. The stationary blade is securedto a support that is coupled resiliently to the cover, said support orthe blade forming a surface for deflecting the path of the paper onleaving the print head.

Other characteristics and advantages appear from the followingdescription of an embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING

Reference is made to the sole accompanying FIGURE which is a sectionview showing an opening print head of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The FIGURE shows only a portion of the printer of the invention. Thehousing in which it is contained comprises a fixed Chassis portion 1which, for example, may belong to the chassis of a machine in which theprinter merely constitutes a part, e.g. a cash register. The chassis 1includes all the means required for fixing the thermal print head 2which essentially comprises a ceramic substrate 3 carrying a row ofheating points 4 and chips 5 for controlling the application of powerthereto. The face of the substrate 3 opposite to that carrying the rowof points and the electronic components carries a radiator 6 fordissipating heat. The print head is hinged about an axis 7 that issecured to the fixed chassis portion 1, and it is urged about the axis 7against a fixed stop 8 by means of a spring 9. The above representationof the thermal head is entirely schematic and does not include all ofthe electrical conductor elements for powering it and for connecting itto the controlling microprocessor.

To the right of the FIGURE, the fixed chassis portion 1 includes acompartment 11 capable of receiving a roll 12 of thermal (i.e.heat-sensitive) paper. This compartment is closed by a cover 13 which,for example, is hinged to the fixed chassis portion 1 about an axis thatis substantially parallel to the row of points 4 (and thus perpendicularto the plane in the FIGURE) and situated on the line 14.

The end of the cover 13 remote from its hinge axis to the fixed chassisportion 1 is fitted with a paper drive roll 15 called a "capstan" whichextends parallel to the hinge axis of the cover 13. This roll isconstrained to rotate with a gear wheel 16 suitable for meshing with acomplementary gear wheel 17 carried by the fixed chassis portion 1 andforming the last gear wheel in a driving assembly (not shown) which iscontrolled in known manner to operate synchronously with printing.

When the cover 13 is opened or closed, the extreme generator line of thepaper drive roll 15 describes a circular arc 18 that interferes with theend of the print head 2. On this circular arc, arrows F and O indicaterespectively the closing direction and the opening direction of thecover 13 when pivoting about its hinge axis. The tip 2a of the head (thetip of the radiator 6) is shaped in such a manner that when the cover isclosed, the paper drive roll 15 rolls over the said tip, thereby causingthe head to pivot about the axis 7 against the force of the spring 9.This shaped surface may be a surface that is inclined or rounded and itseffect is to displace the print head to the left as the paper drive roll15 goes past during closure of the cover 13. The cover reaches itsclosed position when the shaft 15a of the paper drive roll 15 hasreached the bottom of a slot 19 in the fixed chassis portion 1, suchthat the gear wheel 16 meshes with the gear wheel 17. The row of heatingpoints 4 then lies above the line 14, i.e. above the diameter of thepaper drive roll 15 which intersects the hinge axis of the cover 13, inother words behind said line 14 in the closing direction F. As a result,the print head 2 constitutes a kind of latch for holding the cover 13closed.

The printer of the invention also includes a member for cutting thepaper 26 leaving the print head. This cutter member is constituted by arotary blade 20 co-operating with a stationary blade 21. The rotaryblade is carried by the fixed chassis portion 1. above the print head 2.The stationary blade 21 is secured to a support 22 which is hinged ataxis 23 to the cover 13. This support is subjected to the force of aspring 24 urging it to rotate about the axis 23 in an counter-clockwisedirection towards a stop (not shown) which holds the support 22 in adetermined position when the cover 13 is open. Closing the cover bringsthe support 22 into contact with the support 20a for the rotary blade 20so that the edge of the stationary blade 21 crosses the edge of therotary blade 20, with this being done under the pressure from the spring24. The moving blade 20 is rotated by a motor carried by the fixedchassis portion 1. This drive is not shown.

It should be observed that the blades used may be made of a ceramicmaterial, in which case it is important to prevent the two bladesstriking each other when the cover is closed. For this purpose,provision can be made such that while the cover is being closed thesupport 22 (which is made of metal) bears against a portion of thesupport 20a for the blade 20 via a portion lying outside the ceramicstationary blade 21 which portion of the support 20a lies outside saidblade 20 and can be in the form of a cam (an eccentric) so that duringclosure contact between the supports prevents contact between theblades, but after the rotary blade has been put into rotation, theceramic blades come into contact with each other in progressive manner.

Finally, it should be observed that in this FIGURE, that when the cover13 is closed, the stationary blade 21 (or its support) forms a surface25 for deflecting the path of the paper 26 coming from the paper roll 12on leaving the printer. This surface 25 is at an obtuse angle with thetangent to the paper drive roll 15 that passes through the row ofpoints, the value of the angle being about 135°. This disposition isadvantageous since it curves the paper before it goes through thecutting member, with such curvature imparting a degree of "springiness"to the paper which improves the action of the moving blade 20 duringcutting and which tends, after cutting, to keep the paper engagedbetween the two separated blades.

I claim:
 1. A thermal printer for printing on paper, comprising athermal print head having at least one row of heating points, thatcooperates with a paper drive roll by bearing thereagainst, the printhead being carried resiliently by a fixed chassis portion, said paperdrive roll being carried by a cover having a hinge and being hinged tosaid fixed chassis and forming an access to a compartment in the printerfor containing a roll of paper, wherein, when said cover is closed, azone of contact between said paper drive roll and said at least one rowof heating points is situated, in a closing direction of said cover,behind a diameter line of said paper drive roll that passes through saidhinge between said cover and said fixed chassis portion, said print headcomprising a tip portion disposed in front of said at least one row ofheating points, said tip portion being shaped so as to provide acam-forming surface for displacing said print head when reacting againsta force said paper drive roll traveling past said tip portion in saidclosing direction.
 2. A printer according to claim 1, wherein downstreamfrom the print head (2) in a paper travel direction, the fixed chassisportion (1) carries a rotary blade (20) of a paper cutter member, and astationary blade (21) of said paper cutter member is carried by thecover (13) downstream from the paper drive roll (15).
 3. A printeraccording to claim 2, wherein the stationary blade (21) is carried by asupport (22) that is resiliently coupled to the cover (13) and thatpresents, upstream from a cutting edge of the stationary blade (21), inthe paper travel direction, a surface (25) for deflecting a path of thepaper (26), which surface forms an obtuse angle with a tangent to thepaper drive roll (15) at the row of heating points (4).
 4. A printeraccording to claim wherein the support (22) of the stationary blade (21)is hinged to the cover (13).
 5. A printer for printing on paper, theprinter comprising a chassis and a print head that cooperates with apaper drive roll, said print head having a cam-shaped tip forfacilitating displacement thereof when said paper drive roll is pressedagainst said print head; and said chassis comprising a first portion anda second portion which are movable one with respect to the other betweena first position in which said print head and said paper drive roll arespaced apart and a second position in which said print head bearsagainst said paper drive roll, and wherein, downstream from said printhead, in a paper travel direction, said first portion of said chassiscarries a moving blade of a paper cutter member, and said second portionof said chassis carries a stationary blade of said paper cutter member;and a support for carrying said stationary blade, said support beingresiliently coupled to said second portion of said chassis andpresenting, upstream from a cutting edge, in the paper travel direction,a surface for deflecting a path of the paper.
 6. A printer according toclaim 5, wherein the support of the stationary blade is hinged to saidportion of the chassis.
 7. A printer according to claim 5, wherein saidmoving blade is a rotary blade.